1. Field of the Invention
The invention relates generally to security features in printed documents and more specifically to visible authentication patterns in printed documents. The visible authentication patterns can be used to distinguish original printed documents from photocopies of those printed documents, to detect alterations in documents, and to carry hidden and/or visible messages.
2. Description of Related Art
Visible Authentication Patterns (VAPs) can be used to determine whether a printed document has been altered or whether the document is an original or a copy. A VAP is a noisy pattern in a portion of a digital document. It is used to determine the authenticity of a document by comparing a portion of a digital recording made from the analog form with an original digital representation of the portion of the analog form to determine a degree of dissimilarity (or similarity) between the recorded portion and the original digital representation of the portion and using the degree of dissimilarity (or similarity) to determine whether the analog form is an original analog form.
VAPs and their uses are the subject of U.S. patent application Ser. No. 10/514,271, cited above. Further experience with the VAP has resulted in improvements in a number of areas:                Storing information in a VAP without affecting the VAP's property of distinguishing copies from originals;        Using the entropy of a VAP to locate it in an image of a document;        Lessening the aesthetic effect of incorporating a VAP into a document;        Dealing with the fact that a VAP may be subject to a wide variety of printing and scanning processes, and these processes will modify the VAP as it appears on an original analog copy and on a non-original analog form;        Using analogs to the VAP in other applications involving transformations from digital to analog and vice-versa and even in digital-to-digital copying.        
It is an object of the inventions disclosed herein to provide VAPs which are improved with regard to these and other areas.